Doc: libcrmcommon: Note possible flag setting in xml_calculate_changes()

It's not immediately obvious how xml_calculate_changes() might affect
the old_xml argument. Until time allows tracing the function and
creating a full doxygen block, this new comment tells us how old_xml
might be changed.

There doesn't seem to be any other possible change besides setting
pcmk__xf_skip.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>